This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-133335, filed Apr. 27, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a light scanning unit used in a laser printer, a digital copying machine and the like, and more particularly, to a light scanning unit of an over illumination type in which a width of luminous flux incident to a polygon mirror in a main scanning direction (a direction along a direction of rotation of the polygon mirror) is wider than a width of a surface of a reflection surface in the polygon mirror in the main scanning direction.
2. Description of the Related Art
In the laser printer, the digital copying machine and the like corresponding to an image forming apparatus of an electrostatic process copying method in which an electrostatic latent image is formed by using an optical beam and a visible (developer) image is obtained by developing the electrostatic latent image, there can be employed an optical scanning unit which decomposes an image to be output (an original image) into a first direction and a second direction perpendicular to the first direction, and repeatedly outputs, that is, scans an optical beam having a light intensity changed on the basis of an image data in any one of the decomposed first direction and second direction in a substantially linear manner at a predetermined time interval. In this case, an image corresponding to the original image can be obtained by moving a recording medium and a latent image holding body at a predetermined speed in a direction perpendicular to the scanned light beam during a time when the light beam corresponding to one line and the light beam corresponding to the next one line are scanned or during a time when one line is scanned.
In the light scanning unit mentioned above, the direction in which the light beam is scanned, that is, the first direction is generally called as a main scanning direction. Further, the second direction perpendicular to the first direction is generally called as a sub-scanning direction.
The light scanning unit includes a semiconductor laser device corresponding to a light source for emitting the light beam (the laser beam), a first lens group for adjusting cross sectional beam diameters and cross sectional shaped of the light beams emitted from the semiconductor laser device to a predetermined magnitude and shape, a deflecting unit for continuously reflecting the light beams adjusted to the predetermined magnitude and shape by the first lens group in a direction perpendicular to a direction in which the recording medium or the latent image holding body is moved, thereby deflecting (scanning) the light beams, a second lens group for forming an image of the light beam deflected by the deflecting unit at a predetermined position of the recording medium or the latent image holding body, and the like.
The image forming apparatus shifts the recording medium or the latent image holding member for holding the image (latent image) at only one line every time when the image data corresponding to one line is irradiated by the light scanning unit so as to form the aligned image in sequence in the direction in which the image corresponding to one line is orthogonal to the image corresponding to one line. In this case, the image forming apparatus can be classified into a wet type (a liquid type) and a dry type, and a direct type and a copy type (an indirect type) in correspondence to a method of making the latent image visible and whether or not the latent image is directly formed on the recording medium.
In the image forming apparatus and the light scanning unit as mentioned above, a relation expressed by the following formula (1) is established between a process speed (a speed at which the recording medium or the latent holding body is moved), an image resolution (a number of dots per unit length, generally a number of dots per 1 inch), a rotational speed (a rotational number) of a polygon mirror motor of a deflecting unit and a number of surfaces of the polygon mirror of the deflecting unit, when the process speed is set to P (mm/s), the image resolution (the number of the dots per 1 inch) is set to R (dpi), the rotational number of the polygon mirror motor is set to Vr (rpm) and the number of the surfaces of the polygon mirror is set to N.
Pxc3x97R=25.4xc3x97Vrxc3x97N/60xe2x80x83xe2x80x83(1) 
On the basis of the formula (1), it is deemed that the image forming speed and the image resolution are in proportion to the rotational number of the polygon mirror (being generally the same as the rotational number of the polygon mirror motor) and the number of the surfaces of the polygon mirror. Accordingly, in order to make the speed of the image forming apparatus high (increase the image forming speed per the unit time) and make the resolution high (improve the image resolution), it is necessary to increase the number of the surfaces of the polygon mirror or increase the rotational number of the polygon mirror.
In these days, in the light scanning unit of an under illumination type (a generic term at a time of comparing with an over illumination type) utilized in a lot of image forming apparatuses, the width in the main scanning direction of the light beam (the light flux) incident to the polygon mirror (corresponding to the cross sectional beam diameter, and the beam diameter in the main scanning direction in the case that the main scanning direction is different from the sub scanning direction) is limited to be small in comparison with the width in the main scanning direction of an optional reflective surface of the polygon mirror. Accordingly, the light beam guided to each of the reflective surfaces of the polygon mirror is all reflected by the reflective surface.
On the contrary, the cross sectional beam diameter of the light beam guided to the recording medium or the latent image holding body (the image surface) (the beam diameter in the main scanning direction in the case that the main scanning direction is different from the sub scanning direction) is in proportion to an F number Fn of the second lens group (an image forming optical system). The F number Fn is expressed by the formula Fn=f/D in the case that a focal distance of the image forming optical system is set to f, and the diameter in the main scanning direction of the light beam on the optional reflective surface in the polygon mirror is set to D. Accordingly, in order to increase the resolution, when it is intended to reduce the cross sectional beam diameter of the light beam on the subject to be scanned (the image surface), that is, the recording medium or the latent image holding body (a photo conductor), it is necessary to increase the cross sectional beam diameter in the main scanning direction on each of the reflective surfaces of the polygon mirror. Accordingly, when increasing the surface width of each of the reflective surfaces of the polygon mirror and further increasing the surface number of the reflective surfaces, the size of the polygon mirror is made large.
However, in order to rotate the polygon mirror having a large size at a high speed, a large-size motor having a large torque is required. In this case, of course, the motor cost is increased. At the same time, since a sound and a vibration are increased, and a lot of heat is generated, it is necessary to device a countermeasure against them.
As the countermeasure, there is a light scanning unit of an over illumination type. A principle of the light scanning unit using the over illumination is disclosed, for example, in Laser Scanning Notebook (attributed to Leo Beiser, SPIE OPTICAL ENGINEERING PRESS) and the like.
In the light scanning unit of the over illumination type, since the width of the main scanning direction of the optical beam irradiated onto each of the reflective surfaces of the polygon mirror is set to be larger than the width in the main scanning direction of the individual reflective surface of the polygon mirror, it is possible to reflect the light beam on whole surface of each of the reflective surfaces. Accordingly, it is possible to increase the number of the reflective surfaces of the polygon mirror in order to increase the image forming speed and the resolution without increasing the size of the polygon mirror, particularly, the diameter thereof more than necessary.
As mentioned above, a magnitude of a load applied to the polygon motor can be reduced by applying the over illumination to the relation between the cross sectional beam diameter of the light beam irradiated onto the polygon mirror and the width in the main scanning direction of each of the reflective surfaces in the polygon mirror.
Further, even when rotating the polygon mirror at a high speed, a heat generation can be restricted.
Accordingly, a cost of the polygon motor can be reduced. Further, since the surface number of the reflective surfaces of the polygon mirror is increased, whereby a shape of the main scanning surface of the polygon mirror gets close to a complete round, it is possible to reduce the sound, particularly a wind noise and a vibration. In this case, since the sound and the vibration are reduced to a certain level, thereby reducing the number of the parts such as a dust-proof glass and the like which are required for preventing the sound and keep an airtight, it is expected to obtain an advantage that the cost can be further reduced. Further, it is possible to increase a duty cycle.
In the light scanning unit of the over illumination type mentioned above, there is a problem that the width of the light beam deflected by the polygon mirror in the main scanning direction changes in correspondence to a scanning angle although in the light scanning unit of the under illumination type, the width of the light beam deflected by the polygon mirror in the main scanning direction is fixed without relation to the scanning angle (a positional angle).
As mentioned above, in the light scanning unit of the over illumination type, since the F number Fn changes in correspondence to the change of the angle at which the light beam guided to the optional reflective surface of the polygon mirror is reflected on the optional reflective surface of the polygon mirror, that is, the scanning angle, a dispersion (a deviation) is generated in the main scanning direction of the cross sectional beam diameter of the light beam reaching the photo conductor (the image surface). In particular, in the case that the light beam incident to the polygon mirror has an angle with respect to an optical axis of the image forming optical system on the main scanning plane (the light beam is incident from an oblique direction), there is a problem that the dispersion of the cross sectional beam diameters is left-right asymmetrical to the center of the optical axis of the image forming optical system, in connection with the main scanning direction.
In this case, when the light beam is incident from the front surface (near) the main scanning direction onto the optional reflective surface of the polygon mirror in order to reduce the dispersion of the cross sectional beam diameter of the optical beam on the image surface (the photo conductor), there is a case that the light beam reflected (deflected) on the individual reflective surface of the polygon mirror is reflected by the second lens group so as to be returned to the optional reflective surface of the polygon mirror, and then is again reflected toward the second lens group. When the light beam oscillating between the second lens group and the optional reflective surface of the polygon mirror forms the image within the image area of the photo conductor, there is a problem that an image quality of the image is deteriorated.
Further, it is possible to prevent the light beam oscillating between the second lens group and each of the reflective surfaces of the polygon mirror from being generated, by providing with a reflection preventing film (a coating for improving a light transmittance) on the lens surface of the lens included in the second lens group.
However, of course, there is a problem that a cost of a lens simple substance is increased. In this case, in the case that the reflection preventing film is provided on the lens surface of the lens having a low glass transition temperature, there is a risk that a profile irregularity of the lens surface is reduced and an optical performance is deteriorated.
An object of the present invention is to provide a light scanning unit of an over illumination type which can prevent a deterioration of an image quality caused by a light beam reflected on a lens surface of an image forming lens for forming an image of a light beam on a photo conductor being again scanned to the photo conductor through an image forming lens, and can reduce a size of the unit.
The invention is made for the purpose of achieving the object mentioned above, and provided a light scanning unit comprising: a pre-deflection optical system aligning a light flux from a light source so as to form a long linear image in a first direction; a light scanning means scanning the light flux to a predetermined position of a subject to be scanned; and an image forming optical system forming an image of the light flux scanned by the light scanning means on the subject to be scanned, the light flux incident to the light scanning means being wider than a width in the first direction of a single reflective surface of the light scanning means, wherein the image forming optical system includes an optical part having at least one surface which is asymmetrical in the first direction with respect to a center of the optical axis.
Further, the invention provides a light scanning unit comprising: a pre-deflection optical system aligning a light flux from a light source so as to form a long linear image in a first direction; a light scanning means scanning the light flux to a predetermined position of a subject to be scanned; and an image forming optical system forming an image of the light flux scanned by the light scanning means on the subject to be scanned, the light flux incident to the light scanning means being wider than a width in the first direction of a single reflective surface of the light scanning means, wherein the image forming optical system has at least one surface in which its own center is rotated at a predetermined amount with respect to a center of rotation of a surface rotated around an axis in a second direction perpendicular to the first direction, at a position where a longitudinal center is made eccentric or crosses to the optical axis, in the first direction with respect to the optical axis.
Still further, the invention provides a light scanning unit comprising: a pre-deflection optical system aligning a light flux from a light source so as to form a long linear image in a first direction; a light scanning means scanning the light flux to a predetermined position of a subject to be scanned; and an image forming optical system forming an image of the light flux scanned by the light scanning means on the subject to be scanned, the light flux incident to the light scanning means being wider than a width in the first direction of a single reflective surface of the light scanning means, wherein the image forming optical system includes optical parts having at least one surface with a power, and an aspherical surface term of a shape of at least one the surface with the power among the optical parts includes a term Amxe2x89xa00 or Amnxe2x89xa00 in the formula Amxcexa3ym (m=0, 1, 2, 3, . . . ) or Amnxcexa3ymzn (m=0, 1, 2, 3, . . . , n=0, 1, 2, 3, . . . ) when the formula m=2xc3x97k+1 (k=0, 1, 2, 3, . . . ) is established, in the case of setting the first direction of the at least one surface with the power to a y axis, a second direction perpendicular to the first direction to a z axis and a direction in which the light flux moves forward to an x axis.
Furthermore, the invention provides a light scanning unit comprising: a pre-deflection optical system aligning a light flux from a light source so as to form a long linear image in a first direction; a light scanning means scanning the light flux to a predetermined position of a subject to be scanned; and an image forming optical system forming an image of the light flux scanned by the light scanning means on the subject to be scanned, the light flux incident to the light scanning means being wider than a width in the first direction of a single reflective surface of the light scanning means, wherein the image forming optical system includes a lens having a power, the lens having the power has at least one surface accompanying with not surface treatment for restricting a reflection at a time when the light flux is incident, and an incident light to the light scanning means and an optical axis of the image forming optical system have an angle on the main scanning plane or the sub scanning plane.
Moreover, the invention provides a light scanning unit comprising: a pre-deflection optical system aligning a light flux from a light source so as to form a long linear image in a first direction; a light scanning means scanning the light flux to a predetermined position of a subject to be scanned; and an image forming optical system forming an image of the light flux scanned by the light scanning means on the subject to be scanned, the light flux incident to the light scanning means being wider than a width in the first direction of a single reflective surface of the light scanning means, wherein the image forming optical system includes a plurality of optical parts, and a number of the optical parts having a power is equal to or less than 2.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.